K

General Chemistry I: Molecular Geometry and Bonding Theories

General Chemistry I: Molecular Geometry and Bonding Theories

Introduction to Molecular Geometry and Bonding

  • Molecular Geometry: Refers to the general shape of a molecule, which is determined by the relative positions of its atomic nuclei.

  • Theories of Chemical Bonding: Two main theories are used to describe the geometries of molecules in terms of their electronic structures:

    1. Valence Bond Theory

    2. Molecular Orbital Theory

Valence-Shell Electron-Pair Repulsion (VSEPR) Model

  • Determinants of Molecular Shape:

    1. Electron pairs (whether bonding or nonbonding) repel each other.

    2. The shape of a molecule can be predicted by assuming electron pairs are arranged as far apart as possible to minimize repulsions.

  • VSEPR Model Principle: The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.

  • Electron Domains: Represent the directions in which electrons point. This includes single electron pairs, multiple electron pairs (in multiple bonds), or lone pairs. For example, a central atom with four electron domains means four areas where electrons are concentrated.

Electron-Domain Geometries and Predicted Bond Angles

  • The fundamental shapes determined by the number of electron domains around a central atom (A) are:

    • 2 Electron Domains:

      • Arrangement: Linear

      • Electron-Domain Geometry: Linear (AB_2)

      • Predicted Bond Angle: 180^{\circ}

      • Example: CO_2

    • 3 Electron Domains:

      • Arrangement: Trigonal planar

      • Electron-Domain Geometry: Trigonal planar (AB_3)

      • Predicted Bond Angle: 120^{\circ}

      • Example: SO_3

    • 4 Electron Domains:

      • Arrangement: Tetrahedral

      • Electron-Domain Geometry: Tetrahedral (AB_4)

      • Predicted Bond Angle: 109.5^{\circ}

      • Example: CH_4

    • 5 Electron Domains:

      • Arrangement: Trigonal bipyramidal

      • Electron-Domain Geometry: Trigonal bipyramidal (AB_5)

      • Predicted Bond Angles: 120^{\circ} (equatorial) and 90^{\circ} (axial)

      • Example: PCl_5

    • 6 Electron Domains:

      • Arrangement: Octahedral

      • Electron-Domain Geometry: Octahedral (AB_6)

      • Predicted Bond Angle: 90^{\circ}

      • Example: SF_6

Molecular Geometries Based on Electron Domains

  • Once the electron-domain geometry is determined, the molecular geometry is defined by the arrangement of the bonded atoms.

2 Electron Domains (Linear Electron Domain)

  • Molecular Geometry: Linear (if all domains are bonding pairs)

    • Note: If only two atoms are in a molecule, it will always be linear, regardless of electron domain.

3 Electron Domains (Trigonal Planar Electron Domain)

  • Molecular Geometries:

    1. Trigonal Planar: All electron domains are bonding pairs.

    2. Bent: One of the domains is a nonbonding pair.

4 Electron Domains (Tetrahedral Electron Domain)

  • Molecular Geometries:

    1. Tetrahedral: All are bonding pairs.

    2. Trigonal Pyramidal: One is a nonbonding pair.

    3. Bent: Two are nonbonding pairs.

Impact of Nonbonding Pairs and Multiple Bonds on Bond Angles

  • Nonbonding Pairs:

    • Physically larger than bonding pairs.

    • Exert greater repulsions, which tend to compress (decrease) bond angles (e.g., water's 104.5^{\circ} instead of 109.5^{\circ} for a perfect tetrahedron).

  • Multiple Bonds:

    • Double and triple bonds have larger electron domains than single bonds.

    • Exert greater repulsive forces than single bonds, leading to larger bond angles around them and constriction of adjacent single bond angles.

Expanding Beyond the Octet Rule (5 and 6 Electron Domains)

  • Some elements can exceed the octet rule, forming more than four bonds or having more than four electron domains.

5 Electron Domains (Trigonal Bipyramidal Electron Domain)

  • Distinct Positions:

    • Axial: Along the main axis.

    • Equatorial: Around the equatorial plane, where three positions exist at 120-degree angles relative to each other.

Molecular geometry describes a molecule's shape, determined by atomic nuclei positions. It's explained by Valence Bond Theory and Molecular Orbital Theory.

The Valence-Shell Electron-Pair Repulsion (VSEPR) model predicts molecular shape by minimizing repulsions among electron pairs (bonding or nonbonding domains). Key electron-domain geometries and their predicted bond angles are:

  • 2 Electron Domains: Linear (180^{\circ})

  • 3 Electron Domains: Trigonal planar (120^{\circ})

  • 4 Electron Domains: Tetrahedral (109.5^{\circ})

  • 5 Electron Domains: Trigonal bipyramidal (120^{\circ} equatorial, 90^{\circ} axial)

  • 6 Electron Domains: Octahedral (90^{\circ})

Molecular geometry, based on bonded atoms, can differ from electron-domain geometry if lone pairs are present. For example, a tetrahedral electron domain can lead to trigonal pyramidal or bent molecular geometries.

Nonbonding electron pairs are physically larger and exert greater repulsions, compressing bond angles. Multiple bonds also have larger electron domains, causing greater repulsion and affecting adjacent bond angles.

Some elements can expand beyond the octet rule, leading to 5 or 6 electron domains with specific arrangements, such as axial and equatorial positions in trigonal bipyramidal geometry.